Thermal imaging or thermography is a recording process wherein images are generated by the use of imagewise modulated thermal energy.
In thermography two approaches are known:
1. Direct thermal formation of a visible image pattern by imagewise heating of a recording material containing matter that by chemical or physical process changes colour or optical density.
2. Thermal dye transfer printing wherein a visible image pattern is formed by transfer of a coloured species from an imagewise heated donor element onto a receptor element.
Thermal dye transfer printing is a recording method wherein a dye-donor element is used that is provided with a dye layer wherefrom dyed portions or incorporated dyes are transferred onto a contacting receiver element by the application of heat in a pattern normally controlled by electronic information signals.
A survey of "direct thermal" imaging methods is given e.g. in the book "Imaging Systems" by Kurt I. Jacobson-Ralph E. Jacobson, The Focal Press--London and New York (1976), Chapter VII under the heading "7.1Thermography". Thermography is concerned with materials which are substantially not photosensitive, but are sensitive to heat or thermosensitive. Imagewise applied heat is sufficient to bring about a visible change in a thermosensitive imaging material.
Most of the "direct" thermographic recording materials are of the chemical type. On heating to a certain conversion temperature, an irreversible chemical reaction takes place and a coloured image is produced.
As described in "Handbook of Imaging Materials", edited by Arthur S. Diamond--Diamond Research Corporation--Ventura, Calif., printed by Marcel Dekker, Inc. 270 Madison Avenue, New York, N.Y. 10016 (1991), p. 498-499 in thermal printing image signals are converted into electric pulses and then through a driver circuit selectively transferred to a thermal printhead. The thermal printhead consists of microscopic heat resistor elements, which convert the electrical energy into heat via Joule effect. The electric pulses thus converted into thermal signals manifest themselves as heat transferred to the surface of the thermal paper wherein the chemical reaction resulting in colour development takes place.
In a special embodiment of direct thermal imaging a heat-sensitive recording material is used in the form of an electrically resistive ribbon having a multilayered structure in which a carbon-loaded polycarbonate is coated with a thin aluminium film (ref. Progress in Basic Principles of Imaging Systems--Proceedings of the International Congress of Photographic Science K oln (Cologne), 1986 ed. by Friedrich Granzer and Erik Moisar--Friedr. Vieweg & Sohn--Braunschweig/Wiesbaden, FIG. 6. p. 622). Current is injected into the resistive ribbon by electrically addressing a printhead electrode contacting the carbon-loaded substrate, thus resulting in highly localized heating of the ribbon beneath the energized electrode.
The fact that in using a resistive ribbon recording material heat is generated directly in the resistive ribbon and only the travelling ribbon gets hot (not the printheads) an inherent advantage in printing speed is obtained. In applying the thermal printhead technology the various elements of the thermal printhead get hot and must cool down before the printhead can print without cross-talk in a next position.
In another embodiment of direct thermal imaging the recording material is image-wise or pattern-wise heated by means of a modulated laser beam. For example, image-wise modulated infra-red laser light is absorbed in the recording layer in infra-red light absorbing substances converting infra-red radiation into the necessary heat for the imaging reaction.
The imagewise applied laser light has not necessarily to be infrared light since the power of a laser in the visible light range and even in the ultraviolet region can be thus high that sufficient heat is generated on absorption of the laser light in the recording material. There is no limitation on the kind of laser used which may be a gas laser, gas ion laser, e.g. argon ion laser, solid state laser, e.g. Nd:YAG laser, dye laser or semi-conductor laser.
The image signals for modulating the laser beam or current in the micro-resistors of a thermal printhead are obtained directly e.g. from opto-electronic scanning devices or from an intermediary storage means, e.g. magnetic disc or tape or optical disc storage medium, optionally linked to a digital image work station wherein the image information can be processed to satisfy particular needs.
A wide variety of chemical systems has been suggested for use in thermography as described e.g. on page 138 of the above mentioned book of Kurt I. Jacobson et al.
Thermal recording materials are known for the production of black-and-white images as well as for the production of color images.
According to published European patent application 0 599 580 Al a thermal recording sheet suited for thermographic formation of organic dyes contains a leuco dye type chromogenic component consisting of a leuco dye, an organic color developer and a metal chelate type chromogenic component consisting of an electron acceptor and an electron donor, wherein the electron acceptor is a metal double salt of a fatty acid having 16 to 35 carbon atoms, and the electron donor is a polyhydric hydroxy aromatic compound, e.g. a 3,4-dihydroxy benzoic acid amide [see compound(2)].
Particularly useful for black-and-white image formation are thermographic materials the image forming layers of which contain a substantially light-insensitive organic silver salt that at elevated temperature is reduced to silver by a selected organic reducing agent such as hydroquinone, substituted hydroquinones, hindered phenols, pyrogallol, methyl gallate, leuco dyes and the like (ref. U.S. Pat. No. 5,275,932).
Preferred reducing agents for use in combination with said silver salt yield silver images of high optical density, and maintain there reducing property on storage without giving rise to substantial fog.
For high quality image reproduction further preference is given to reducing agents having the above properties in association with the capability to provide silver images with as good as possible colour neutrality having a more pleasing appearance and contrast.
The obtaining of good colour neutrality is particularly problematic when using in thermal recording a thermal print head of which the heat supplied by each micro-resistor is very intensive during a very short heating time so that locally relatively high temperatures are reached that have a "browning" influence on the colour of the silver formed in the reduction process.